In the well-known Bayer process, hot concentrated sodium hydroxide solution is used to attack Bauxite. The action of the sodium hydroxide solution is to solubilize alumina and thus separate it from the other oxides present in the mined bauxite. In the process of dissolving alumina with sodium hydroxide to form aluminates, organics in the bauxite are attacked and degraded into various compounds, such as the degradation of humic acids into oxalates and other organic moieties. The oxalates are soluble in the Bayer liquor. Over time, the recirculation of the Bayer liquor used in the processing of bauxite accumulates these impurities and when the concentration of the impurities reach their limits of supersaturation, they drop out of the solution as solids. A particular concern is with sodium oxalate which precipitates in the form of needles on the aluminum hydroxide seeds. These fine needles act as seeds for the precipitation of the aluminum hydroxide product, causing an increase of fine particles that have to be returned to the Bayer process for reprocessing. This significantly affects the overall efficiency of the Bayer process in isolating aluminum hydroxide from the bauxite.
The sodium oxalates are therefore undesirable and must be removed from the Bayer liquor when its concentration approaches the limit of supersaturation. The crude sodium oxalate is removed by evaporating t increase the sodium hydroxide concentration, which decreases the solubility of sodium oxalate, crystallizing the sodium oxalate, and removing the crystals by decantation, filtration and other techniques. The sodium oxalates are hazardous to the environment and the public, so that they cannot be readily discarded and must therefore be treated before release to the environment.
It is known that oxalic acid can be recovered from the crude sodium oxalate of the Bayer process by reacting the slurry containing the sodium oxalates with lime at 90.degree. C. The reaction produces a calcium oxalate which is precipitated and readily separated from the slurry. The calcium oxalate can then be reacted with 96% sulfuric acid at elevated temperatures to produce a calcium sulfate byproduct and oxalic acid which can be recovered by crystallization of the oxalic acid from the solution. This technique is disclosed in Western German Patent Publication No. DE 2,553,870. This type of process, however, is complicated and consumes considerable lime, sulfuric acid, disposal of calcium sulfate byproduct, dangers in handling of concentrated sulfuric acid and is an energy intensive process. It is therefore desirable to develop a process for treating the crude sodium oxalates of the Bayer process, derive oxalic acid and sodium hydrogen oxalate from the oxalates of sufficient purity to be useful industrially.
Alternative techniques for treating of sodium oxalate, albeit for different purposes to upgrade technical grade sodium oxalate, is disclosed in USSR Patent 401131 issued May 5, 1976. The patent discloses an electrolysis process for upgrading technical grade sodium oxalate. The process is carried out in a fourchamber electrolytic cell where the chambers are separated by ion exchange membranes. In the process of electrolysis, the sodium ions of the sodium oxalate are exchanged for hydrogen ions to prepare oxalic acid of high purity. Iron cathode and graphite anode are used in establishing the electrochemical reaction.
Polish Patent 129059 issued Jun. 28, 1985 discloses a method of regenerating a cation exchanger used in the process of purifying technical grade sodium oxalate to prepare oxalic acid. The patent states that it is known to use a cation exchanger to convert sodium oxalate into sodium hydrogen oxalate and oxalic acid. To assist in the dissolution of the sodium oxalate in a hot solution, oxalic acid is included. The patent discloses the technique for regenerating the column used in preparing oxalic acid by the ion exchange treatment of the sodium oxalate. Such regeneration is carried out with a 7% nitric acid solution followed by a washing with demineralized water at 70.degree. to 90.degree. C. and a subsequent wash with water containing 2% to 5% of nitric acid.